Many practical systems operate over a wide dynamic range. Such systems include, for example, systems to monitor and control audio, atmospheric pressure, temperature, visual light intensity, automotive velocity, mechanical rotation rate, and seismic activity. Many such systems are measured or controlled by electronics. To provide a system operating over a very wide dynamic range, the electronics must possess a high degree of complexity and/or accuracy to be useful over the full operating range of interest. In general, cost of the electronics must be traded against accuracy.
Hearing amplification systems (e.g., for the hearing impaired) typically provide a desirable non-linear transfer function to logarithmically compress a large input dynamic range into a smaller output dynamic range. For analog logarithmic compression it is highly desirable that, a) the compression transfer function be highly predictable, b) the compression transfer function be highly accurate, c) the compression transfer function be operable over a wide input dynamic range, and d) the compression transfer function be electronically programmable.
For audio systems, it is further desirable to control the time rate (or time constant) at which the logarithmic compression is active. If the time constant (also known generally in the audiology field as “attack rate” and “release rate”) is either too fast or too slow, distortion of the audio signal occurs. Therefore, it is desirable to implement analog logarithmic compression with a predictable and controllable time constant.